This invention relates to a system for indirectly injecting intelligence on a carrier wave and relates generally to the method and apparatus described in U.S. Pat. Nos. 4,106,007 and 4,218,655, the disclosures of which are incorporated herein by reference. As described in those patents, it is known that a modulation voltage can be superimposed on a power system voltage, for example, at specified locations on the power system voltage to cause wave shape perturbations in the carrier wave. In the embodiment described hereinafter, the carrier wave is the voltage wave of an electrical power distribution system. Those skilled in the art will recognize the wider applicability of the inventive principles disclosed hereinafter. An example of such perturbation is shown in FIG. 1A of the drawings, labelled "prior art". The perturbation caused by the injected voltage is such that a time interval along successive cycles of the carrier wave can be altered. That is to say, with reference to FIG. 1, T.sub.1 does not equal T.sub.2 when measured at a certain threshold voltage defined as .DELTA.V. A desired signal is defined in the above-referenced patents as .vertline..tau..vertline. =.vertline.T.sub.1 -T.sub.2..vertline.
Several methods have been used in the past to generate the modulation voltage. The technology employed in the above-referenced patents is to insert the modulation voltage directly in series with the source voltage to obtain the desired perturbations on the bus voltage of the distribution system. For ease of signal detection at a receiving point, the threshold voltage .DELTA.V should be kept low. This implies that the peaks of the modulation voltage should be in the vicinity of the zero crossing of the power system voltage. Typically, one full cycle of a.c. modulation voltage is injected at the neutral of a four-wire system, and such a modulation scheme is shown in FIG. 2, where it is labelled as "prior art". In the three-phase, four-wire neutral grounded system shown in FIG. 2, the modulation voltage is injected between the neutral point of the power transformer and the grounding point. Under normal operating conditions, the modulation circuit carries only the zero sequence current of the system due to system unbalance. Even in the system of FIG. 2, however, line-to-ground faults produce very high zero sequence fault current components. Because the modulation voltage is inserted directly onto the distribution line, the modulator circuit must be designed to withstand the occurrence of high current surges. In addition, the modulation voltage has a zero sequence nature and line-to-line voltages are not modulated. An advantage of the system just described is that the modulator circuit is close to ground potential so current surge protection can be accomplished in a number of known ways. While current surge protection is expensive, it is less expensive with neutral injection techniques than with other previously applied signalling methods. This type of modulation is known in the art as "neutral injection".
In order to reach line-to-line customers on the distribution network of a utility, for example, on three-phase systems, the modulation signal preferably should have dominant positive and negative sequence components. This implies that the modulation signal should not appear on all three phases simultaneously at equal strength and phase relationship. In order to obtain the desired signal for such line-to-line customers, a system of "phase injection" is used. A typical system for phase injection is shown in FIG. 3 and labelled as "prior art". In this technique, the modulation signals are injected in series with the source voltage of only one phase. In most standard three-phase transformers, the signal voltage is injected into the outgoing lead of one phase winding by means of a coupling transformer. Since the coupling transformer carries the full bus current, it must be designed so that at fault conditions, it can handle the phase fault current. While this type of transformer design is expensive, the use of a coupling transformer provides isolation between the modulation circuit and the high voltage on the bus.
Direct neutral and phase injection techniques require that a portion of the modulation circuit be inserted directly in the path of the power flow during both normal and abnormal conditions experienced on the distribution system. Consequently, these techniques require relatively expensive components for the modulation circuit. This is particularly true for neutral injection systems where fault currents can be extremely high and for phase injection systems operating at high distribution voltages where even the normal load currents are very high.
While both phase and neutral injection systems work well for their intended purpose, they have proved to be at a cost disadvantage with other known types of power distribution communication systems because of their inherent higher cost. The invention described hereinafter permits utilization of the advantages of a power wave carrier distribution system at a significantly lower cost by utilizing indirect injection of the modulation voltage. In addition, because indirect insertion is employed, a number of variables that enable fine tuning of the modulation imposed on the carrier wave can be utilized. The resulting system is lower in cost and attains equal if not better, discernible intelligence transmissions on a distribution system.
One of the objects of this invention is to provide a method and apparatus for transmitting intelligence over a distribution system.
Another object of this invention is to provide a highly reliable modulation method and apparatus for inserting intelligence on a character wave.
Another object of this invention is to provide a low cost method and apparatus for inserting a modulation wave indirectly on a distribution system.
Another object of this invention is to provide a modulation system which incorporates a number of controllable elements for controlling at least the amplitude of a modulation wave prior to its insertion on a distribution system.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.